1. Field of the Invention
The present invention relates to a propylene-ethylene block copolymer, a method for producing it, a resin composition containing the copolymer, and a blow-molded article of the copolymer. More precisely, the invention relates to a propylene-ethylene block copolymer having good heat resistance and good drawdown resistance and having well-balanced rigidity and impact resistance, especially well-balanced rigidity and low-temperature barrier resistance; to an efficient method for producing the copolymer; to a resin composition containing the copolymer; and to a blow-molded article of the copolymer or the resin composition, which is especially favorable to large-size parts of automobiles such as bumpers, etc.
2. Description of the Related Art
As a rule, polypropylene resins have characteristics of low specific gravity, high rigidity, good dimensional stability and good heat resistance. As being general-purpose resins, therefore, they are molded into articles of desired shapes in various molding methods of, for example, extrusion molding, injection molding, blow molding, etc. Above all, blowmolding is much employed for molding large-size structural members such as typically automobile parts, since molds for it are inexpensive and since its process can be simplified by integrating the process steps. However, blow-molded articles of ordinary polypropylene resins are not always satisfactory in point of the moldability, especially the drawdown resistance, and of the balance of rigidity and impact resistance, especially the balance of rigidity and low-temperature barrier resistance. Therefore, various attempts have been made for solving the problem with polypropylene resins by employing multi-stage polymerization for producing the resins and by adding a nucleating agent to the resins (e.g., Japanese Patent Publication No. 74264/1991, Japanese Patent Laid-Open No. 213547/1988). In particular, for large-size blow-molded articles, polypropylene resins are combined with polyethylene resins for improving their drawdown resistance, and inorganic fillers such as talc and the like are added thereto for compensating for the reduction in the rigidity of the resin blends. However, these result in increasing the weight of the resin moldings.
We, the present inventors have previously found out a technique of improving the rigidity and the drawdown resistance of large-size blow-molded articles of polypropylene resins (WO96/02381). With the recent requirements of weight reduction (for example, wall thickness reduction) and weather resistance with automobile materials, desired is further improving the rigidity and the impact resistance, especially the low-temperature barrier resistance of polypropylene resins.